US9212412B2ActiveUtilityPatentIndex 57
Lean duplex stainless steel excellent in corrosion resistance and toughness of weld heat affected zone

Assignee: OIKAWA YUUSUKEPriority: Mar 26, 2008Filed: Mar 26, 2009Granted: Dec 15, 2015
Est. expiryMar 26, 2028(~1.7 yrs left)· nominal 20-yr term from priority
Inventors:OIKAWA YUUSUKE URASHIMA HIROSHI TSUGE SHINJI INOUE HIROSHIGE MATSUHASHI RYO
C21D 8/02C22C 38/02C22C 38/18C22C 38/42C22C 38/001C22C 38/08C22C 38/12C22C 38/58C22C 38/06C21D 2211/005C22C 38/002C21D 1/20C21D 2211/001C22C 38/40C22C 38/04C22C 38/48C22C 38/46C22C 38/14C22C 38/38Y02E60/10Y02P10/20
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Claims
Abstract

The present invention provides a lean duplex stainless steel able to suppress the drop in corrosion resistance and toughness of a weld heat affected zone comprising, by mass %, C: 0.06% or less, Si: 0.1 to 1.5%, Mn: 2.0 to 4.0%, P: 0.05% or less, S: 0.005% or less, Cr: 19.0 to 23.0%, Ni: 1.0 to 4.0%, Mo: 1.0% or less, Cu: 0.1 to 3.0%, V: 0.05 to 0.5%, Al: 0.003 to 0.050%, O: 0.007% or less, N: 0.10 to 0.25%, and Ti: 0.05% or less, having a balance of Fe and unavoidable impurities. An Md30 value is 80 or less, an Ni-bal is −7.1 to 4, an austenite phase area percentage is 40 to 70%, and a 2×Ni+Cu is 3.5 or more: Md30=551−462×(C+N)−9.2×Si−8.1×Mn−29×(Ni+Cu)−13.7×Cr−18.5×Mo−68×Nb; Ni-bal=(Ni+0.5Mn+0.5Cu+30C+30N)−1.1(Cr+1.5Si+Mo+W)+8.2 and N(%)≦0.37+0.03×(Ni-bal).
Claims

exact text as granted — not AI-modified
The invention claimed is:  
     
       1. A lean duplex stainless steel having corrosion resistance and toughness of a weld heat affected zone comprising, by mass %,
 C: 0.06% or less, Si: 0.1 to 1.5%, Mn: 2.0 to 4.0%, P: 0.05% or less, S: 0.005% or less, Cr: 19.0 to 23.0%, Ni: 1.0 to 4.0%, Mo: 1.0% or less, Cu: 0.1 to 3.0%, V: 0.05 to 0.5%, Al: 0.003 to 0.050%, O: 0.007% or less, N: 0.10 to 0.25%, and Ti: 0.05% or less, and having a balance of Fe and unavoidable impurities, wherein 
 an Md30 value expressed by formula &lt;1&gt; is 80 or less, 
 an Ni-bal expressed by formula &lt;2&gt; is −7.1 to −4, 
 the Ni-bal and the N content satisfy formula &lt;3&gt;, 
 the lean duplex stainless steel has an austenite phase area percentage of 40 to 70%, and 
 the lean duplex stainless steel has a value of 2×Ni+Cu of 3.5 or more:
   Md30=551−462×(C+N)−9.2×Si−8.1×Mn−29×(Ni+Cu)−13.7×Cr−18.5×Mo−68×Nb  &lt;1&gt;;
 
   Ni-bal=(Ni+0.5Mn+0.5Cu+30C+30N)−1.1(Cr+1.5Si+Mo+W)+8.2  &lt;2&gt;;
 
   N(%)≦0.37+0.03×(Ni-bal)  &lt;3&gt;;
 
 
 wherein, in the above formulas, the element symbols represent the content of the elements in mass %. 
 
     
     
       2. The lean duplex stainless steel as set forth in  claim 1 , further comprising, by mass %, Nb: 0.02 to 0.15%, wherein the product of the Nb and N content in mass %, Nb×N, is 0.003 to 0.015. 
     
     
       3. The lean duplex stainless steel as set forth in  claim 1 , further comprising, by mass %, one or more of no more than 0.0050% Ca, no more than 0.0050% Mg, no more than 0.050% REM, and no more than 0.0040% B. 
     
     
       4. The lean duplex stainless steel as set forth in  claim 1 , further comprising, by mass %, Nb: 0.02 to 0.15%, wherein the product of the Nb and N content in mass %, Nb×N, is 0.003 to 0.015, and one or more of no more than 0.0050% Ca, no more than 0.0050% Mg, no more than 0.050% REM, and no more than 0.0040% B. 
     
     
       5. The lean duplex stainless steel as set forth in  claim 1 , further comprising, by mass %, Co: 0.02 to 1.00%. 
     
     
       6. The lean duplex stainless steel as set forth in  claim 1 , further comprising, by mass %, Nb: 0.02 to 0.15%, wherein the product of the Nb and N content in mass %, Nb×N, is 0.003 to 0.015, Co: 0.02 to 1.00%. 
     
     
       7. The lean duplex stainless steel as set forth in  claim 1 , further comprising, by mass %, Nb: 0.02 to 0.15%, wherein the product of the Nb and N content in mass %, Nb×N, is 0.003 to 0.015, Co: 0.02 to 1.00%, and one or more of no more than 0.0050% Ca, no more than 0.0050% Mg, no more than 0.050% REM, and no more than 0.0040% B. 
     
     
       8. The lean duplex stainless steel as set forth in  claim 1 , further comprising, by mass %, Mg: 0.0001 to 0.0050%, wherein a product of f N , Ti content in mass %, and N content in mass %, f N ×Ti×N, is 0.00004 or more, and a product of Ti content and N content in mass %, Ti×N, is 0.008 or less:
 wherein, f N  is a value satisfying formula &lt;4&gt;:
   log 10   f   N =−0.046×Cr−0.02×Mn−0.011×Mo+0.048×Si+0.007×Ni+0.009×Cu  &lt;4&gt;
 
 
 wherein, in the above formula, the element symbols represent the content of the elements in mass %. 
 
     
     
       9. The lean duplex stainless steel as set forth in  claim 1 , further comprising, by mass %, one or more of Zr≦0.03%, Ta≦0.1%, W≦1.0%, and Sn≦0.1%. 
     
     
       10. The lean duplex stainless steel as set forth in  claim 1 , wherein following
 (i) a temperature elevation from room temperature to 1300° C. in 15 seconds, (ii) retention at 1300° C. for 5 seconds, (iii) isothermal cooling from 1300° C. to 900° C. in 15 seconds, (iv) isothermal cooling from 900° C. to 400° C. in 135 seconds, and (v) rapid cooling from 400° C. to room temperature, simulating a heat history received by the steel at the time of welding, a Cr extract residue of the steel has a value of 0.025% or less and a CRN value, as determined in the following formula &lt;5&gt;, of 0.5 or more:
     CRN =([Cr]/104)/{([Cr]/104)+([V]/51)+([Nb]/93)+([B]/11)}  &lt;5&gt;
 
 
 where, [Cr], [V], [Nb], and [B] all express extract residue amounts of the elements in mass %. 
 
     
     
       11. The lean duplex stainless steel as set forth in  claim 2 , wherein following:
 (i) a temperature elevation from room temperature to 1300° C. in 15 seconds, (ii) retention at 1300° C. for 5 seconds, (iii) isothermal cooling from 1300° C. to 900° C. in 15 seconds, (iv) isothermal cooling from 900° C. to 400° C. in 135 seconds, and (v) rapid cooling from 400° C. to room temperature, simulating a heat history received by the steel at the time of welding, a Cr extract residue of the steel has a value of 0.025% or less and a CRN value, as determined in the following formula &lt;5&gt;, of 0.5 or more:
     CRN =([Cr]/104)/{([Cr]/104)+([V]/51)+([Nb]/93)+([B]/11)}  &lt;5&gt;
 
 
 where, [Cr], [V], [Nb], and [B] all express extract residue amounts of the elements in mass %. 
 
     
     
       12. The lean duplex stainless steel as set forth in  claim 3 , wherein following:
 (i) a temperature elevation from room temperature to 1300° C. in 15 seconds, (ii) retention at 1300° C. for 5 seconds, (iii) isothermal cooling from 1300° C. to 900° C. in 15 seconds, (iv) isothermal cooling from 900° C. to 400° C. in 135 seconds, and (v) rapid cooling from 400° C. to room temperature, simulating a heat history received by the steel at the time of welding, a Cr extract residue of the steel has a value of 0.025% or less and a CRN value, as determined in the following formula &lt;5&gt;, of 0.5 or more:
     CRN =([Cr]/104)/{([Cr]/104)+([V]/51)+([Nb]/93)+([B]/11)}  &lt;5&gt;
 
 
 where, [Cr], [V], [Nb], and [B] all express extract residue amounts of the elements in mass %. 
 
     
     
       13. The lean duplex stainless steel as set forth in  claim 4 , wherein following:
 (i) a temperature elevation from room temperature to 1300° C. in 15 seconds, (ii) retention at 1300° C. for 5 seconds, (iii) isothermal cooling from 1300° C. to 900° C. in 15 seconds, (iv) isothermal cooling from 900° C. to 400° C. in 135 seconds, and (v) rapid cooling from 400° C. to room temperature, simulating a heat history received by the steel at the time of welding, a Cr extract residue of the steel has a value of 0.025% or less and a CRN value, as determined in the following formula &lt;5&gt;, of 0.5 or more:
     CRN =([Cr]/104)/{([Cr]/104)+([V]/51)+([Nb]/93)+([B]/11)}  &lt;5&gt;
 
 
 where, [Cr], [V], [Nb], and [B] all express extract residue amounts of the elements in mass %. 
 
     
     
       14. The lean duplex stainless steel as set forth in  claim 5 , wherein following:
 (i) a temperature elevation from room temperature to 1300° C. in 15 seconds, (ii) retention at 1300° C. for 5 seconds, (iii) isothermal cooling from 1300° C. to 900° C. in 15 seconds, (iv) isothermal cooling from 900° C. to 400° C. in 135 seconds, and (v) rapid cooling from 400° C. to room temperature, simulating a heat history received by the steel at the time of welding, a Cr extract residue of the steel has a value of 0.025% or less and a CRN value, as determined in the following formula &lt;5&gt;, of 0.5 or more:
     CRN =([Cr]/104)/{([Cr]/104)+([V]/51)+([Nb]/93)+([B]/11)}  &lt;5&gt;
 
 
 where, [Cr], [V], [Nb], and [B] all express extract residue amounts of the elements in mass %. 
 
     
     
       15. The lean duplex stainless steel as set forth in  claim 6 , wherein following:
 (i) a temperature elevation from room temperature to 1300° C. in 15 seconds, (ii) retention at 1300° C. for 5 seconds, (iii) isothermal cooling from 1300° C. to 900° C. in 15 seconds, (iv) isothermal cooling from 900° C. to 400° C. in 135 seconds, and (v) rapid cooling from 400° C. to room temperature, simulating a heat history received by the steel at the time of welding, a Cr extract residue of the steel has a value of 0.025% or less and a CRN value, as determined in the following formula &lt;5&gt;, of 0.5 or more:
     CRN =([Cr]/104)/{([Cr]/104)+([V]/51)+([Nb]/93)+([B]/11)}  &lt;5&gt;
 
 
 where, [Cr], [V], [Nb], and [B] all express extract residue amounts of the elements in mass %. 
 
     
     
       16. The lean duplex stainless steel as set forth in  claim 7 , wherein following:
 (i) a temperature elevation from room temperature to 1300° C. in 15 seconds, (ii) retention at 1300° C. for 5 seconds, (iii) isothermal cooling from 1300° C. to 900° C. in 15 seconds, (iv) isothermal cooling from 900° C. to 400° C. in 135 seconds, and (v) rapid cooling from 400° C. to room temperature, simulating a heat history received by the steel at the time of welding, a Cr extract residue of the steel has a value of 0.025% or less and a CRN value, as determined in the following formula &lt;5&gt;, of 0.5 or more:
     CRN =([Cr]/104)/{([Cr]/104)+([V]/51)+([Nb]/93)+([B]/11)}  &lt;5&gt;
 
 
 where, [Cr], [V], [Nb], and [B] all express extract residue amounts of the elements in mass %. 
 
     
     
       17. The lean duplex stainless steel as set forth in  claim 8 , wherein following:
 (i) a temperature elevation from room temperature to 1300° C. in 15 seconds, (ii) retention at 1300° C. for 5 seconds, (iii) isothermal cooling from 1300° C. to 900° C. in 15 seconds, (iv) isothermal cooling from 900° C. to 400° C. in 135 seconds, and (v) rapid cooling from 400° C. to room temperature, simulating a heat history received by the steel at the time of welding, a Cr extract residue of the steel has a value of 0.025% or less and a CRN value, as determined in the following formula &lt;5&gt;, of 0.5 or more:
     CRN =([Cr]/104)/{([Cr]/104)+([V]/51)+([Nb]/93)+([B]/11)}  &lt;5&gt;
 
 
 where, [Cr], [V], [Nb], and [B] all express extract residue amounts of the elements in mass %. 
 
     
     
       18. The lean duplex stainless steel as set forth in  claim 9 , wherein following:
 (i) a temperature elevation from room temperature to 1300° C. in 15 seconds, (ii) retention at 1300° C. for 5 seconds, (iii) isothermal cooling from 1300° C. to 900° C. in 15 seconds, (iv) isothermal cooling from 900° C. to 400° C. in 135 seconds, and (v) rapid cooling from 400° C. to room temperature, simulating a heat history received by the steel at the time of welding, a Cr extract residue of the steel has a value of 0.025% or less and a CRN value, as determined in the following formula &lt;5&gt;, of 0.5 or more:
     CRN =([Cr]/104)/{([Cr]/104)+([V]/51)+([Nb]/93)+([B]/11)}  &lt;5&gt;
 
 
 where, [Cr], [V], [Nb], and [B] all express extract residue amounts of the elements in mass %. 
 
     
     
       19. The lean duplex stainless steel as set forth in  claim 1 , further comprising, by mass %, Co: 0.02 to 1.00% and one or more of no more than 0.0050% Ca, no more than 0.0050% Mg, no more than 0.050% REM, and no more than 0.0040% B. 
     
     
       20. The lean duplex stainless steel as set forth in  claim 1 , further comprising, by mass %, Nb: 0.02 to 0.15%, wherein the product of the Nb and N content in mass %, Nb×N, is 0.003 to 0.015, and Mg: 0.0001 to 0.0050%, wherein a product of f N , Ti content in mass %, and N content in mass %, f N ×Ti×N, is 0.00004 or more, and a product of Ti content and N content in mass %, Ti×N, is 0.008 or less:
 wherein, f N  is a value satisfying formula &lt;4&gt;:
   log 10   f   N =−0.046×Cr−0.02×Mn−0.011×Mo+0.048×Si+0.007×Ni+0.009×Cu  &lt;4&gt;
 
 
 wherein, in the above formula, the element symbols represent the content of the elements in mass %. 
 
     
     
       21. The lean duplex stainless steel as set forth in  claim 1 , further comprising, by mass %, one or more of no more than 0.0050% Ca, no more than 0.050% REM, and no more than 0.0040% B, and Mg: 0.0001 to 0.0050%, wherein a product of f N , Ti content in mass %, and N content in mass %, f N ×Ti×N, is 0.00004 or more, and a product of Ti content and N content in mass %, Ti×N, is 0.008 or less:
 wherein, f N  is a value satisfying formula &lt;4&gt;:
   log 10   f   N =−0.046×Cr−0.02×Mn−0.011×Mo+0.048×Si+0.007×Ni+0.009×Cu  &lt;4&gt;
 
 
 wherein, in the above formula, the element symbols represent the content of the elements in mass %. 
 
     
     
       22. The lean duplex stainless steel as set forth in  claim 1 , further comprising, by mass %, Co: 0.02 to 1.00% and Mg: 0.0001 to 0.0050%, wherein a product of f N , Ti content in mass %, and N content in mass %, f N ×Ti×N, is 0.00004 or more, and a product of Ti content and N content in mass %, Ti×N, is 0.008 or less:
 wherein, f N  is a value satisfying formula &lt;4&gt;:
   log 10   f   N =−0.046×Cr−0.02×Mn−0.011×Mo+0.048×Si+0.007×Ni+0.009×Cu  &lt;4&gt;
 
 
 wherein, in the above formula, the element symbols represent the content of the elements in mass %. 
 
     
     
       23. The lean duplex stainless steel as set forth in  claim 1 , further comprising, by mass %, one or more of Zr≦0.03%, Ta≦0.1%, W≦1.0%, Sn≦0.1%, and Mg: 0.0001 to 0.0050%, wherein a product of f N , Ti content in mass %, and N content in mass %, f N ×Ti×N, is 0.00004 or more, and a product of Ti content and N content in mass %, Ti×N, is 0.008 or less:
 wherein, f N  is a value satisfying formula &lt;4&gt;:
   log 10   f   N =−0.046×Cr−0.02×Mn−0.011×Mo+0.048×Si+0.007×Ni+0.009×Cu  &lt;4&gt;
 
 
 wherein, in the above formula, the element symbols represent the content of the elements in mass %. 
 
     
     
       24. The lean duplex stainless steel as set forth in  claim 19 , further comprising, by mass %, Mg: 0.0001 to 0.0050%, wherein a product of f N , Ti content in mass %, and N content in mass %, f N ×Ti×N, is 0.00004 or more, and a product of Ti content and N content in mass %, Ti×N, is 0.008 or less:
 wherein, f N  is a value satisfying formula &lt;4&gt;:
   log 10   f   N =−0.046×Cr−0.02×Mn−0.011×Mo+0.048×Si+0.007×Ni+0.009×Cu  &lt;4&gt;
 
 
 wherein, in the above formula, the element symbols represent the content of the elements in mass %. 
 
     
     
       25. The lean duplex stainless steel as set forth in  claim 19 , further comprising, by mass %, one or more of Zr≦0.03%, Ta≦0.1%, W≦1.0%, and Sn≦0.1%. 
     
     
       26. The lean duplex stainless steel as set forth in  claim 19 , further comprising, by mass %, Nb: 0.02 to 0.15%, wherein the product of the Nb and N content in mass %, Nb×N, is 0.003 to 0.015, one or more of Zr≦0.03%, Ta≦0.1%, W≦1.0%, and Sn≦0.1%, and Mg: 0.0001 to 0.0050%, wherein a product of f N , Ti content in mass %, and N content in mass %, f N ×Ti×N, is 0.00004 or more, and a product of Ti content and N content in mass %, Ti×N, is 0.008 or less:
 wherein, f N  is a value satisfying formula &lt;4&gt;:
   log 10   f   N =−0.046×Cr−0.02×Mn−0.011×Mo+0.048×Si+0.007×Ni+0.009×Cu  &lt;4&gt;
 
 
 wherein, in the above formula, the element symbols represent the content of the elements in mass %. 
 
     
     
       27. The lean duplex stainless steel as set forth in  claim 19 , further comprising, by mass %, Nb: 0.02 to 0.15%, wherein the product of the Nb and N content in mass %, Nb×N, is 0.003 to 0.015 and Mg: 0.0001 to 0.0050%, wherein a product of f N , Ti content in mass %, and N content in mass %, f N ×Ti×N, is 0.00004 or more, and a product of Ti content and N content in mass %, Ti×N, is 0.008 or less:
 wherein, f N  is a value satisfying formula &lt;4&gt;:
   log 10   f   N =−0.046×Cr−0.02×Mn−0.011×Mo+0.048×Si+0.007×Ni+0.009×Cu  &lt;4&gt;
 
 
 wherein, in the above formula, the element symbols represent the content of the elements in mass %. 
 
     
     
       28. The lean duplex stainless steel as set forth in  claim 19 , further comprising, by mass %, Nb: 0.02 to 0.15%, wherein the product of the Nb and N content in mass %, Nb×N, is 0.003 to 0.015 and one or more of Zr≦0.03%, Ta≦0.1%, W≦1.0%, and Sn≦0.1%. 
     
     
       29. The lean duplex stainless steel as set forth in  claim 19 , further comprising, by mass %, one or more of Zr≦0.03%, Ta≦0.1%, W≦1.0%, and Sn≦0.1%, and Mg: 0.0001 to 0.0050%, wherein a product of f N , Ti content in mass %, and N content in mass %, f N ×Ti×N, is 0.00004 or more, and a product of Ti content and N content in mass %, Ti×N, is 0.008 or less:
 wherein, f N  is a value satisfying formula &lt;4&gt;:
   log 10   f   N =−0.046×Cr−0.02×Mn−0.011×Mo+0.048×Si+0.007×Ni+0.009×Cu  &lt;4&gt;
 
 
 wherein, in the above formula, the element symbols represent the content of the elements in mass %. 
 
     
     
       30. The lean duplex stainless steel as set forth in  claim 19 , wherein following:
 (i) a temperature elevation from room temperature to 1300° C. in 15 seconds, (ii) retention at 1300° C. for 5 seconds, (iii) isothermal cooling from 1300° C. to 900° C. in 15 seconds, (iv) isothermal cooling from 900° C. to 400° C. in 135 seconds, and (v) rapid cooling from 400° C. to room temperature, simulating a heat history received by the steel at the time of welding, a Cr extract residue of the steel has a value of 0.025% or less and a CRN value, as determined in the following formula &lt;5&gt;, of 0.5 or more:
     CRN =([Cr]/104)/{([Cr]/104)+([V]/51)+([Nb]/93)+([B]/11)}  &lt;5&gt;
 
 
 where, [Cr], [V], [Nb], and [B] all express extract residue amounts of the elements in mass %. 
 
     
     
       31. The lean duplex stainless steel as set forth in  claim 24 , wherein following:
 (i) a temperature elevation from room temperature to 1300° C. in 15 seconds, (ii) retention at 1300° C. for 5 seconds, (iii) isothermal cooling from 1300° C. to 900° C. in 15 seconds, (iv) isothermal cooling from 900° C. to 400° C. in 135 seconds, and (v) rapid cooling from 400° C. to room temperature, simulating a heat history received by the steel at the time of welding, a Cr extract residue of the steel has a value of 0.025% or less and a CRN value, as determined in the following formula &lt;5&gt;, of 0.5 or more:
     CRN =([Cr]/104)/{([Cr]/104)+([V]/51)+([Nb]/93)+([B]/11)}  &lt;5&gt;
 
 
 where, [Cr], [V], [Nb], and [B] all express extract residue amounts of the elements in mass %. 
 
     
     
       32. The lean duplex stainless steel as set forth in  claim 25 , wherein following:
 (i) a temperature elevation from room temperature to 1300° C. in 15 seconds, (ii) retention at 1300° C. for 5 seconds, (iii) isothermal cooling from 1300° C. to 900° C. in 15 seconds, (iv) isothermal cooling from 900° C. to 400° C. in 135 seconds, and (v) rapid cooling from 400° C. to room temperature, simulating a heat history received by the steel at the time of welding, a Cr extract residue of the steel has a value of 0.025% or less and a CRN value, as determined in the following formula &lt;5&gt;, of 0.5 or more:
     CRN =([Cr]/104)/{([Cr]/104)+([V]/51)+([Nb]/93)+([B]/11)}  &lt;5&gt;
 
 
 where, [Cr], [V], [Nb], and [B] all express extract residue amounts of the elements in mass %. 
 
     
     
       33. The lean duplex stainless steel as set forth in  claim 26 , wherein following:
 (i) a temperature elevation from room temperature to 1300° C. in 15 seconds, (ii) retention at 1300° C. for 5 seconds, (iii) isothermal cooling from 1300° C. to 900° C. in 15 seconds, (iv) isothermal cooling from 900° C. to 400° C. in 135 seconds, and (v) rapid cooling from 400° C. to room temperature, simulating a heat history received by the steel at the time of welding, a Cr extract residue of the steel has a value of 0.025% or less and a CRN value, as determined in the following formula &lt;5&gt;, of 0.5 or more:
     CRN =([Cr]/104)/{([Cr]/104)+([V]/51)+([Nb]/93)+([B]/11)}  &lt;5&gt;
 
 
 where, [Cr], [V], [Nb], and [B] all express extract residue amounts of the elements in mass %. 
 
     
     
       34. The lean duplex stainless steel as set forth in  claim 27 , wherein following:
 (i) a temperature elevation from room temperature to 1300° C. in 15 seconds, (ii) retention at 1300° C. for 5 seconds, (iii) isothermal cooling from 1300° C. to 900° C. in 15 seconds, (iv) isothermal cooling from 900° C. to 400° C. in 135 seconds, and (v) rapid cooling from 400° C. to room temperature, simulating a heat history received by the steel at the time of welding, a Cr extract residue of the steel has a value of 0.025% or less and a CRN value, as determined in the following formula &lt;5&gt;, of 0.5 or more:
     CRN =([Cr]/104)/{([Cr]/104)+([V]/51)+([Nb]/93)+([B]/11)}  &lt;5&gt;
 
 
 where, [Cr], [V], [Nb], and [B] all express extract residue amounts of the elements in mass %. 
 
     
     
       35. The lean duplex stainless steel as set forth in  claim 28 , wherein following:
 (i) a temperature elevation from room temperature to 1300° C. in 15 seconds, (ii) retention at 1300° C. for 5 seconds, (iii) isothermal cooling from 1300° C. to 900° C. in 15 seconds, (iv) isothermal cooling from 900° C. to 400° C. in 135 seconds, and (v) rapid cooling from 400° C. to room temperature, simulating a heat history received by the steel at the time of welding, a Cr extract residue of the steel has a value of 0.025% or less and a CRN value, as determined in the following formula &lt;5&gt;, of 0.5 or more:
     CRN =([Cr]/104)/{([Cr]/104)+([V]/51)+([Nb]/93)+([B]/11)}  &lt;5&gt;
 
 
 where, [Cr], [V], [Nb], and [B] all express extract residue amounts of the elements in mass %. 
 
     
     
       36. The lean duplex stainless steel as set forth in  claim 29 , wherein following:
 (i) a temperature elevation from room temperature to 1300° C. in 15 seconds, (ii) retention at 1300° C. for 5 seconds, (iii) isothermal cooling from 1300° C. to 900° C. in 15 seconds, (iv) isothermal cooling from 900° C. to 400° C. in 135 seconds, and (v) rapid cooling from 400° C. to room temperature, simulating a heat history received by the steel at the time of welding, a Cr extract residue of the steel has a value of 0.025% or less and a CRN value, as determined in the following formula &lt;5&gt;, of 0.5 or more:
     CRN =([Cr]/104)/{([Cr]/104)+([V]/51)+([Nb]/93)+([B]/11)}  &lt;5&gt;
 
 
 where, [Cr], [V], [Nb], and [B] all express extract residue amounts of the elements in mass %. 
 
     
     
       37. The lean duplex stainless steel as set forth in  claim 3 , further comprising, by mass %, one or more of Zr≦0.03%, Ta≦0.1%, W≦1.0%, and Sn≦0.1%. 
     
     
       38. The lean duplex stainless steel as set forth in  claim 5 , further comprising, by mass %, one or more of Zr≦0.03%, Ta≦0.1%, W≦1.0%, and Sn≦0.1%. 
     
     
       39. The lean duplex stainless steel as set forth in  claim 1 , further comprising, by mass %, one or more selected from the group consisting of
 a) Nb: 0.02 to 0.15%, wherein the product of the Nb and N content in mass %, Nb×N, is 0.003 to 0.015%; 
 b) one or more of no more than 0.0050% Ca, no more than 0.0050% Mg, no more than 0.050% REM, and no more than 0.0040% B; 
 c) Co: 0.02 to 1.00%; and 
 d) one or more of Zr≦0.03%, Ta≦0.1%, W≦1.0%, and Sn≦0.1%. 
 
     
     
       40. The lean duplex stainless steel as set forth in  claim 1 , wherein Ti is not added. 
     
     
       41. The lean duplex stainless steel as set forth in  claim 39 , wherein Ti is not added. 
     
     
       42. A lean duplex stainless steel having corrosion resistance and toughness of a weld heat affected zone comprising, by mass %,
 C: 0.06% or less, Si: 0.1 to 1.5%, Mn: 2.0 to 4.0%, P: 0.05% or less, S: 0.005% or less, Cr: 19.0 to 23.0%, Ni: 1.55 to 4.0%, Mo: 1.0% or less, Cu: 0.1 to 3.0%, V: 0.05 to 0.5%, Al: 0.003 to 0.050%, O: 0.007% or less, N: 0.10 to 0.25%, and Ti: 0.05% or less, and having a balance of Fe and unavoidable impurities, wherein 
 an Md30 value expressed by formula &lt;1&gt; is 80 or less, 
 an Ni-bal expressed by formula &lt;2&gt; is −7.1 to −4, 
 the Ni-bal and the N content satisfy formula &lt;3&gt;, 
 the lean duplex stainless steel has an austenite phase area percentage of 40 to 70%, and 
 the lean duplex stainless steel has a value of 2×Ni+Cu of 3.5 or more:
   Md30=551−462×(C+N)−9.2×Si−8.1×Mn−29×(Ni+Cu)−13.7×Cr−18.5×Mo−68×Nb  &lt;1&gt;;
 
   Ni-bal=(Ni+0.5Mn+0.5Cu+30C+30N)−1.1(Cr+1.5Si+Mo+W)+8.2  &lt;2&gt;;
 
   N(%)≦0.37+0.03×(Ni-bal)  &lt;3&gt;;
 
 
 wherein, in the above formulas, the element symbols represent the content of the elements in mass %. 
 
     
     
       43. The lean duplex stainless steel as set forth in  claim 42 , wherein Ti is not added.
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